When using RTK or differential GPS technologies, considering the Base has a clear unobstructed view, what view of the sky is required to be able to get an accurate fix with the rover when is moving (boat on water)?

For Trimble Real-Time Kinematic (RTK) and rovers a minimum of 4 satellites must be tracked to obtain RTK coordinate solutions. With 5 common satellites necessary for RTK initialization.

The Global Position System (GPS) is designed so at least 5 satellites are above the horizon at all times, but there can be 8 or more during some times of the day. See here for more information on this topic.

It is difficult to answer 'what view of the sky is required?, but in the most basic sense enough so that the base and rovers can pick up at least 4 satellites. The amount of sky required may differ depending on where you are in the world or what time of day it is.

There is work being done in Hells Canyon, ID which is a very deep canyon (North America's deepest river gorge) that uses RTK GPS as part of their setup when collecting bathymetric/topographic data with multibeam and single beam sonar. Despite the depth of the canyon in many places it is fairly open which allows the ability to obtain signals from 4 satellites.

In contrast the work done in the Grand Canyon by the Grand Canyon Monitoring and Research Center (GCMRC) is unable to leverage RTK GPS because of the lack of satellite coverage in the deep canyon. The sheer cliffs and relatively narrow nature of the Grand Canyon is perhaps more impactful on RTK than how deep it is. To gain more insight for how they overcome these limitations to collect topographic data it may be worthwhile for you to visit the GCMRC website here.

Thank you very much for the detailed comment. We are working on a bathymetry project, the reservoir has quite steep hills all around (60 degrees from horizontal). In the past we had issues with getting a GPS fix, however, we avoided this by using a total station instead. We plan to use "bottom tracking" to navigate in areas with poor signal. However, our client has asked that we loosely quantify the area that we will be using the les acurite "bottom tracking" technology.
– Alan ClarkeOct 31 '15 at 16:02

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Use project planner to determine availability Windows and block out times with low PDOP.
– Brad NesomNov 1 '15 at 17:39

@AlanClarke As a new user please take the Tour as an introduction to our focussed Q&A format. Rather than respond to requests for clarification using more comments, you can use the edit button to revise your question with those additional details.
– PolyGeo♦Nov 1 '15 at 23:38

You can use mission planning tools where for a defined location you can input obstructions using bearing and elevation. You will then be able to see how these obstructions affect satellite availability and the GDOP (Geometric Dilution Of Precision). GDOP value is very important to know if the geometry of the visible satellites is good to achieve a reliable position.

Google "Trimble mission planning" for the software and make sure you download the latest almanac.